Recently I have seen a bit of talk on motor-based catapults and was wondering how you set them up. Is it essentially just a fast-moving arm? How do you do the force calculations for distance?
Thanks
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The most notable example I can think of for a motor driven catapult is Boom Done Ri3D in 2014
Here’s a video that has a high level overview of how it works
That’s really all that I can think of, wish I could provide more information
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The best resource we found was this white paper from Team 230: https://www.chiefdelphi.com/uploads/default/original/3X/c/4/c4fd84c75983c0cd32572ca0f4ea36f3d4f962e5.pdf
It covers the physics and how to calculate approximate gear ratios for your motor and projectile.
My team used the white paper to determine that with the ~24" arm we could package in the long KOP chassis and a single NEO motor a 25:1 ratio was adequate. Testing revealed decent performance but also only needed about 50% duty cycle, so we swapped to 36:1 and dropped current draw while gaining consistency with it now running 75% duty cycle.
We use 2 catapults in parallel. Each arm is directly attached to the output shaft of a MaxPlanetary gearbox on a 2x2" tube tower that mounts to the top of the KOP chassis. The arm is 1x1x0.05" square tube with a 3D printed “jai alai” style ball holder. Control is simply commanding a desired duty cycle (we actually use voltage control mode) until reaching a preset soft limit (~6 motor rotations) at which point brake mode will engage and release the ball. It makes for a very low energy shot and of our ~200 attempted high hub shots from the fender last weekend only 2 bounced out. I can probably dig up some slow-mo shots we took during troubleshooting that illustrate operation pretty well.
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Look at 2014:
A lot of teams used a motor to wind up & store potential energy in something, then released it withe a “grenade pin”. A shifting gearbox with the high speed side gears removed was common.
This is for a purely hypothetical bot but I would be kind of scared of consistency and durability of a grenade pin style.
Does anyone see M----- Based and automatically read Mentor-Based? Because I read “How do Mentor-Based catapults work” which is a similar but distinct thread.
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We designed a bot with arms powered by four CIM in Power Ups that threw the cube onto the scale. Used I think 36:1 Andymark planetaries. We basically did velocity control pid and then the release of the cube was purely timing based. Wasn’t perfect and battery voltage had to be almost maxed out to work.
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Look up a choo choo mechanism. That’s another way of doing it that I don’t see people do often
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It’s against the rules to have a mentor riding around on the bot acting as a catapult. I’m pretty sure it’s rule G909 
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This is just the pair of links that get wound up and then the whole thing over-centers and it shoots… Correct?
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I know of them. Choo choo mechanisms work best for lots of the same shot. I wanna see what a catapult can do from anywhere on the field which is where using a motor driven direct drive would be better as I think it would allow for more difference in the shot.
Yeah, one nice feature of direct drive is that it is sort of a constant acceleration device (constant torque). As you move further from the goal you need a more horizontal trajectory and thus later release point in the catapult arc. Since acceleration is constant you will naturally get a greater release velocity with greater total arm travel. We actually had to reduce motor power when testing at 10 feet compared to against the fender because the velocity increase was too much.
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